This application is based upon and claims the benefit of Japanese Patent Application No. 11-207715 filed on Jul. 22, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a semiconductor physical quantity sensor having a semiconductor oscillator such as an acceleration sensor or an angular velocity sensor, which is adopted to detect a moving state of a moving body such as a vehicle, an airplane, or a robot.
2. Description of the Related Art
Generally, this kind of semiconductor physical quantity sensor is formed by a micro-machine processing technique such as etching performed on a semiconductor substrate. The semiconductor physical quantity sensor typically has a base, an oscillator that oscillates in a specific direction, and several beam portions connecting the oscillator and the base. A physical quantity (angular velocity, acceleration, or the like) is detected based on oscillation of the oscillator.
For instance, when the semiconductor quantity sensor is an angular velocity sensor, the oscillator is drive-oscillated in a specific direction. An angular velocity applied to the sensor is detected based on oscillation (detection oscillation) that is produced by a Coriolis force in a direction perpendicular to the drive oscillation direction and a rotational axis of the angular velocity.
When the semiconductor quantity sensor is an acceleration sensor, acceleration applied to the oscillator is detected based on oscillation (detection oscillation) that is produced in a specific direction.
However, it is inevitable for the conventional physical quantity sensor to have processing errors (for instance, by etching). The processing errors can induce unnecessary oscillation in addition to the drive oscillation and detection oscillation described above, resulting in measurement errors. The inventors considered that the unnecessary oscillation might be produced due to the beam portions connecting the oscillator and the base. That is, in the conventional physical quantity sensor as disclosed in JP-A-6-123631, each of the beam portions is composed of one pole-like beam having small rotational rigidity. Therefore, each beam can be distorted easily during the oscillation of the oscillator to induce unnecessary oscillation.
The present invention has been made in view of the above problems. An object of the present invention is to provide a semiconductor physical quantity sensor having a beam portion that is structured with high rotational rigidity not to prevent oscillation of an oscillator.
According to the present invention, a beam portion is composed of first and second beams that are disposed in parallel with and separately from each other, and abridge portion connecting the first and second beams. This beam portion can have increased rotational rigidity without preventing oscillation of an oscillator.
Preferably, each of the first and second beams has a first part extending in a direction approximately perpendicular to an oscillation direction in which the oscillator oscillates to detect a physical quantity, and a second part extending in the oscillation direction and connected to the bridge portion. More preferably, a width defined between the second part of the first beam and the second part of the second beam is larger than that defined between the first part of the first beam and the first part of the second beam. Accordingly, a difference in displacement between the first and second beams caused by a difference in length between the first parts of the first and second beams can be absorbed easily not to prevent the oscillation of the oscillator.
Preferably, the first and second beams defines a gap therebetween, and the bridge portion divides the gap into first and second gaps each having a generally L shape so that the first and second gaps are symmetrical with each other with respect to the bridge portion. More preferably, the bridge portion has a width that is approximately equal to that of the first and second beams. This feature further facilitates the oscillation of the oscillator.